Thermosetting unsaturated polyester resin based molding compounds are well known and consist of an unsaturated polyester resin, an ethylenically unsaturated copolymerizable monomer, inert mineral fillers, fibrous reinforcing fillers and a catalyst which initiates the cross-linking reaction between the copolymerizable monomer and the unsaturated polyester resin at a chosen molding temperature. They are commonly used in many physical forms two of which are bulk molding compounds (BMC) and sheet molding compounds (SMC). In bulk molding compounds, the fibrous reinforcing filler which is normally about 0.25 inch in length, is mechanically mixed with unsaturated resin. In sheet molding compounds, the fibrous reinforcing filler which is normally about 0.5 to 2 inches in length, is present as chopped fibers deposited on a supporting carrier which are impregnated with the unsaturated resin system, giving rise to sheets of molding compound in which the fibers have not been subjected to the degrading action of conventional molding compound mixers.
The use of unsaturated polyester resins in the molding of glass fiber reinforced or other fiber reinforced products enjoys broad application in the manufacture of automotive, industrial and home products.
Polyester molding compounds are described in U.S. Pat. Nos. 3,940,350, 3,974,124, 3,227,665, 3,701,748 and 3,948,716.
Unfortunately, polyester resins are inherently brittle and whenever unsaturated polyester resins are used as a matrix, then the toughening of the matrix is important. Normal industry practice is to add a flexibilizer to the resin composition which often results in a significant reduction in the mechanical properties. This reduction in mechanical properties is unacceptable for many product applications. It is desirable to toughen unsaturated polyester resin compositions without significant adverse effects on other important properties such as cure rate, maturation and strength.